The present invention relates to a transfer belt unit used in an electrophotographic image forming apparatus and also relates to an image forming apparatus provided with such a transfer belt unit.
An electrophotographic image forming apparatus forms a toner image on a surface of an image bearing member such as a photoreceptor drum or an intermediate transfer belt, and transfers the toner image onto a sheet by a transfer means. Some image forming apparatuses are provided with a transfer belt unit including an endless transfer belt as the transfer means.
The transfer belt unit is configured to stretch the endless transfer belt over a plurality of rollers including a driving roller and a transfer roller. The driving roller receives rotation supplied from a driving source. The transfer belt is pressed against the image bearing member with the transfer belt held therebetween in a transfer position, and forms a transfer electric field by means of a transfer bias being applied from a power supply portion. The transfer belt moves along a predetermined circulation path by rotation of the driving roller. The transfer belt of which circulation path includes a part of a sheet feed path which passes through a transfer position, has a function of a conveyor belt to convey a sheet via the transfer position.
In order to correctly transfer a toner image on the surface of the image bearing member onto a sheet, it is necessary to synchronize a conveying speed of the sheet in the transfer position with a moving speed of the surface of the image bearing member, and it is also necessary to stably move the transfer belt at a constant speed. A conveyor belt which has a function of conveying a sheet within the image forming apparatus is formed in a plane shape, using resin as a material and setting an outer peripheral surface and an inner peripheral surface as mirror surfaces, so that a slip easily occurs between the conveyor belt and the driving roller, which may cause a case where it is difficult to move the conveyor belt stably at a constant speed.
For this reason, as disclosed in Japanese Patent Laid-Open Publication No. 2001-192136, for example, some conventional conveyor belts made of resin have a roughened inner peripheral surface in contact with a peripheral surface of the driving roller. The configuration is regarded to be capable of preventing the slip by increasing friction between the conveyor belt and the driving roller and also moving the conveyor belt at a constant speed.
Thus, roughening the inner peripheral surface of the transfer belt is considered to be capable of: eliminating the slip between the transfer belt and the driving roller, moving the transfer belt while synchronizing with a peripheral speed of the driving roller, and also conveying a sheet stably at a constant speed.
However, in a case of roughening the inner peripheral surface of the transfer belt, keeping the outer peripheral surface of the transfer belt pressed against the image bearing member as a mirror surface may cause vibration of the transfer belt by friction between the image bearing member and the outer peripheral surface of the transfer belt, the friction being caused by errors of the diameter of the driving roller or the thickness of the transfer belt. If such a state is left as it is, a sheet which passes through the transfer position will move slightly forward and backward, which may prevent a toner image from being transferred correctly from the image bearing member onto the sheet, and thereby causes deterioration of an image formed on the sheet. Such a problem may also be caused in a case where the inner peripheral surface of the transfer belt is a mirror surface.
In view of the foregoing problems, it is an object of the present invention is to provide a transfer belt unit capable of preventing occurrence of vibration of the transfer belt due to friction with an image bearing member and correctly transferring a toner image onto a sheet conveyed in a transfer position through the transfer belt, and to also provide an image forming apparatus provided with such a transfer belt unit.